The present invention relates to a thermal head and a method of manufacturing the same and, more particularly, to a thermal head suitable for constituting a multithermal head used in a color printer, a method of manufacturing the same, and a multithermal head constituted by the thermal head.
As conventional image printing systems, an electrophotograpic system, an electrostatic system, a dry discharge system, an ink jet system, a thermal sensitive system, a thermal transfer system, and the like are well known. Efforts have been made not only to improve the quality of characters and figures formed by these systems, but also to decrease the running and manufacturing costs of them.
The conventional thermal sensitive system and thermal transfer system are compact and have a simple mechanical structure. In addition, these systems have a maintenance-free structure, do not make much noise and do not give off unpleasant smell. Therefore, they have been getting popular and have rapidly prevailed. In particular, the thermal transfer system allows color printing in principle. Various attempts have been made to enable the system to print color image. However, no feasible techniques for color printing have been developed.
In the conventional thermal transfer system, an ink film, which consists of a polymeric base film and solid ink coated on the base film and having thermal melting or sublimation properties, overlays recording normal paper. A thermal head is urged against the ink film. The head has a plurality of heating elements. These elements are selectively turned on/off in accordance with predetermined information while the ink film and the thermal head are moved relative to each other. The ink is transferred from the base film to the recording paper to print an image corresponding to the information. The following various techniques, which the thermal transfer system can employ to achieve color printing, are available:
(1) To use cyan, magenta, yellow and black ink films and four thermal heads corresponding to the four color films.
(2) To use one thermal head and a single ink film with the four colors sequentially coated on the base film along its longitudinal direction (the film has four-color lateral stripes regularly repeated on the base film along its longitudinal direction). Recording paper is reciprocated four times to record a single color image.
(3) To use four thermal heads and one ink film with four-color lateral stripes repeated on the base film along the longitudinal direction, and to move recording paper is only one direction.
Technique (1) has disadvantages in large size and high cost of a thermal transfer color printer. Although technique (2) is advantageous since a small printer suffices, color misregistration occurs and complex mechanisms is required since recording paper must be reciprocated four times. Technique (3) is promising over techniques (1) and (2), if the size of a multithermal head with four thermal heads is decreased.
In the conventional thermal head, the heating elements and an IC for driving these elements are mounted on the same surface region of a substrate. When four thermal heads of this type are arranged close to each other, thus providing a multithermal head, four groups of heating elements and four ICs must be provided in the same planar area. The planar area must inevitably be large. Furthermore, since each driver IC is higher than the heating elements, the recording paper cannot be fed parallel to the planar surface between the four groups of heating elements. A mechanism must be used which waves the recording paper between the four groups of heating elements to prevent the paper from colliding with the four driver ICs. Obviously, this mechanism is complex and very expensive.
Conventional thermal heads for solving the above problems are described in Japanese Utility Model Disclosure No. 57-193545 and Japanese Patent Disclosure Nos. 58-92576 and 57-93171. In these prior art thermal heads, a plurality of heating elements are arranged in a predetermined pattern on the upper surface of a base having a triangular or semi-circular cross section. A plurality of lead wires are connected at one ends to the heating elements. The other end of each lead wire extends on a side surface perpendicular to a tangent at a vertex of the top surface and is electrically connected to the heating element driver IC fixed on the side surface.
Even the above prior art thermal heads have the following defect. Since photoetching is adapted to form the heating elements and lead wires on the surface of the base, it is difficult to etch curved portions, shoulders or legs of the base if a portion of the base, on which the heating elements and lead wires to be formed, has a semi-circle, rectangle or trapezoid cross section. This is due to the following reason. When light is applied through a mask to a photoresist film, thereby performing contact or proximity exposure in normal photoetching, the resolution at such portion does not satisfy the requirements for a high density of the heating elements and lead wires since a distance between the mask and a photoresist film is variable at the curved portions, shoulders or legs. The thermal head described in Japanese Utility Model Disclosure No. 57-193545 has a columnar base, therefore, it is difficult to mount such a thermal head in a printer. In the thermal head described in Japanese Patent Disclosure No. 58-92576, since the lead wires, which have one ends electrically connected to a corresponding one of heating elements, have the other ends located on the inclined surface of the base having a triangular cross section, wire bonding between the other end of each lead wire and a heating element driver IC is difficult.